Alterations both in cellular protooncogenes (dominant events) and in tumor suppressor genes (recessive events) appear to be key steps in tumor induction. The multifunctional T antigen of SV40 possesses a number of transformation-related activities, including complex formation with at least two tumor suppressor proteins, pRb and p53. Substantial evidence suggests that these cellular proteins act to regulate the cell cycle. The possibility that tumor virus transforming proteins such as T antigen act to disrupt the normal function of both p53 and pRB provides an opportunity to explore the action of these cellular proteins in growth regulation of cells within the animal. Our ultimate goal using transgenic mice is to determine the role of each T antigen sub-activity, including pRB and p53-binding, in the tumorigenesis of a variety of cell types. As a first step in analyzing the targeting of specific cells, we focused on the response of the choroid plexus epithelium (CPE) to T antigen. We have shown that T antigen is sufficient to subvert normal growth regulation in these cells and that its action is cell-autonomous. Moreover, a preliminary analysis of mutant forms of T antigen in this system indicates that multiple T antigen activities, including pRB-binding, but possibly not p53-binding, are critical for the tumorigenic phenotype. Thus, we are in an excellent position to explore, in detail, the molecular mechanism of T antigen action in the CPE. Furthermore, these studies provide a framework for a similar preliminary study in other specific cell types. Thus, this proposal aims to: I) Determine the role of pRB in CPE tumorigenesis; II) Determine the role of p53 in CPE tumorigenesis; III) Characterize the role of unidentified T antigen activities in CPE tumorigenesis; IV) Determine the cooperative effects of T antigen sub-activities in the CPE; V) Explore the influence of cell type on the mechanism of T antigen-mediated tumorigenesis. Experiments are planned which target the expression of mutant forms of T antigen, as well as other key tumor virus proteins, to the CPE. In a more preliminary study, targeted expression of select mutant T antigens will be used to explore the response of a limited number of cell types (colonic, retinal, and lymphoid). The design here is patterned after the initial studies which we have already performed in choroid plexus.